feat: 添加PWA manifest、更新品牌图标配色、添加打印样式

frontend/public/favicon.svg

@@ -1,8 +1,8 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32">
   <defs>
     <linearGradient id="g" x1="0%" y1="0%" x2="100%" y2="100%">
-      <stop offset="0%" stop-color="#4F46E5"/>
-      <stop offset="100%" stop-color="#8B5CF6"/>
+      <stop offset="0%" stop-color="#0f766e"/>
+      <stop offset="100%" stop-color="#10b981"/>
     </linearGradient>
   </defs>
   <rect width="32" height="32" rx="8" fill="url(#g)"/>

frontend/public/manifest.json

@@ -0,0 +1,18 @@
+{
+  "name": "智教助手 - 教师专属AI备课工具",
+  "short_name": "智教助手",
+  "description": "面向K12教师的AI备课与教学创作平台，一句话生成互动课件、教学动画、课堂游戏与智能测练",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#f5f7fa",
+  "theme_color": "#0f766e",
+  "orientation": "any",
+  "icons": [
+    {
+      "src": "/favicon.svg",
+      "sizes": "any",
+      "type": "image/svg+xml",
+      "purpose": "any maskable"
+    }
+  ]
+}